WASHINGTON (Reuters) -- U.S. regulators have expanded the scope of an investigation into complaints that airbags on certain Ford Motor Co F-150 pickups deployed without warning and caused dozens of injuries.

The National Highway Traffic Safety Administration made documents public this week that show the agency is now looking at model year 2004-06 trucks, covering an estimated 1.56 million vehicles. That is triple the number identified when the agency launched a preliminary review of 2005 pickups last fall.

Safety engineers have upgraded the investigation of the best-selling F-150 to an engineering analysis, which can precede a recall. The government gave no indication that a recall was the next step.

Ford said it was too soon to speculate on an outcome but was cooperating with the investigation, which covers more than 300 complaints to the automaker from F-150 owners that the driver's side airbag deployed without warning and not as a result of a crash.

More than 60 people reported injuries, including cuts to the face, arms and hands, NHTSA said. Two owners reported losing consciousness after being struck by the airbag.

Ford believes the problem is most likely caused by an electrical short in the airbag/horn assembly. Ford made design changes for 2007 model year F-150s.